Adjustable nonlinearly transmissive optical device

ABSTRACT

A nonlinear optical device which is useful because of its intensity-dependent transmissivity characteristic for Q switching or mode locking lasers and also as an optical limiter is disclosed. A combination of adjustable optical elements each of which causes a known effect on the polarization of radiation passing therethrough forms the device; one of these elements is a dielectric material which rotates the major axis of elliptically polarized radiation in an amount proportional to the intensity of the radiation, producing an intensity sensitive transmissive characteristic. The saturation parameters of the devices are variable and devices having either a saturable absorber or a saturable transmitter characteristic are described.

United States Glenn ADJUSTABLE NONLINEARLY TRANSMISSIVE OPTICAL DEVICE[72] Inventor: William H. Glenn, Vernon, Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

22] Filed: April 5,1971

21 Appl.No.: 131,321

[52] U.S. Cl ..350/157, 350/147 [5 1] Int. Cl .I ..G02f 1/24 [58] Fieldof Search ..350/147, 150, 157, I60

Maker et al., lntensity- Dependent Changes in the Refractive Index ofLiquids Phys. Rev. Lett. Vol. 12, No. 18 (May 4, 1964) PP- 507- 509McWane et al., New Measurements of Intensity-De- I I Oct. 24, 1972pendent Changes in the Refractive Index of Liquids App. Phys. Lett. Vol.8, No. 11 (June l, 1966) pp. 278- 279 Primary Examiner-David SchonbergAssistant ExaminerPaul R. Miller Attorney-Anthony .l. Criso [57]ABSTRACT A nonlinear optical device which is useful because of itsintensity-dependent transmissivity characteristic for Q switching ormode locking lasers and also as an optical limiter is disclosed. Acombination of adjustable optical elements each of which causes a knowneffect on the polarization of radiation passing therethrough forms thedevice; one of these elements is a dielectric material which rotates themajor axis of elliptically polarized radiation in an amount proportionalto the intensity of the radiation, producing an intensity sensitivetransmissive characteristic. The saturation parameters of the devicesare variable and devices having either a saturable absorber or asaturable transmitter characteristic are described.

4 Claims, 3 Drawing Figures ADJUSTABLE NONLINEARLY TRANSMISSIVE OPTICALDEVICE BACKGROUND OF THE INVENTION field of an optical beam by anonlinearly transmissive optical device. The invention herein describedwas made in the course of or under a contract or subcontract thereunderwith the Department of the Air Force.

2. Description of the Prior Art In the past, various non-linearmaterials called bleachable dyes have been developed, particularly foruse with lasers. The most common use for these materials is with opticalradiation (any electromagnetic radiation having a wavelength very muchsmaller than the characteristic dimensions of the physical objectsinvolved) as a saturable absorber or as a saturable transmitter. Ableachable dye generally comprises an organic molecule such ascryptocyanine, various phthalocyanines and Eastman Kodak 9740 and 9860,suspended in a solid or liquid host such as ethanol, chlorobenzene,dichloroethane, or polymethylmethacrylate, and has a useful absorptioncharacteristic at the wavelength corresponding to that of an incidentlaser. For example, in a 0 switching application a dye cell is placed ina laser cavity containing a gain medium. When the cavity gain exceedsits losses, the laser intensity increases causing an increasedtransmissivity through the dye cell; as the intensity increases, thecavity losses decrease and effectively increase the gain of the cavity.The higher gain causes a still further increased intensity, whichrepeats the indicated sequence and the net result is a Q switched laser.There is an analogous descriptive model for the mode locking of lasersalthough the exact mechanisms involved are not as clearly understood; asa laser pulse passes through a dye, the transmissivity of the dyeincreases and this increased transmissivity persists for a period afterthe pulse has left the dye. To mode lock a laser, the persistence of theincreased transmissivity must be minimized and in any case, thepersistance cannot exceed the time a light pulse requires to make around trip through the cavity involved.

One of the drawbacks presently limiting the usefulness of the knownbleachable dyes in laser work is the relatively narrow span ofwavelengths over which a dye is useful; more specifically, a particulardye must be selected for each different laser since these materialsexhibit their nonlinear characteristics over a relatively limitedwavelength span.

Further, many of the known dyes having nonlinear properties have beenfound to be chemically unstable; for example, the dye may undergochemical change due to ambient light or laser light impingement andtherefore exhibit a relatively short storable lifetime and/or arelatively short useful lifetime.

Also, many of the dyes under consideration are expensive to use. Theactual chemical compositions of some of the commonly used dyes are knownonly by the manufacturers of these products, and the persons using suchdyes cannot prepare them from staple chemical supplies but must purchasethem already fully prepared.

A severe limitation in the use of known bleachable dyes occurs becausethe optical properties of such dyes cannot be readily changed. Forexample, each saturable absorber has a characteristic saturableintensity (the intensity at which the absorption coefficient is reducedto one-half the value at zero intensity) which is dependent upon the dyemolecule and the dye solvent and therefore is not easily varied once agiven dye is prepared.

Nonlinear optical devices are used as both absorbers and transmitters ofelectromagnetic radiation. As is known in the art, a saturable absorberproduces a transmissivity which increases with increasing intensity ofthe radiation due to an equalization of the population in the upper andlower energy states corresponding to the absorption line.Correspondingly, a saturable transmitter provides a transmissivity whichdecreases with increasing intensity of the radiation, due to thepopulation of the excited energy states of the medium which exhibit alarger absorption than the ground energy state.

SUMMARY OF THE INVENTION A principle object of the present invention isto provide an optical device which has an'adjustable transmissivity ofelectromagnetic radiation which is intensity-dependent.

According to the present invention, a dielectric material which rotatesthe polarization ellipse of an elliptically polarized wave in an amountdependent upon the intensity of the wave, is combined with polarizersand wave retardation plates to provide an optical device havingtransmissive characteristics which are both nonlinear with respect toincident electromagnetic radiation intensity and adjustable over a rangeof possible variations. In one preferred embodiment, the presentinvention is a transmission device in which a linearly polarized inputbeam of electromagnetic radiation is passed in sequence through a firstwave plate, a dielectric material, a second wave plate, and a polarizerto produce a linearly polarized output beam. A second preferredembodiment is a reflection device in which an input beam ofelectromagnetic energy having any polarization characteristic is passedthrough the abovedescribed preferred embodiment in the reverseddirection before striking a mirror and being reflected back to repassthrough the same components and produce a linearly polarized output beamexiting said polarizer.

A principal advantage of the present invention is the ability to varythe transmissivity in a given nonlinear optical device; once adielectric material is selected, the functional dependence of thetransmissivity of the overall device on the intensity of theelectromagnetic radiation passing therethrough can be varied over a widerange of characteristic forms (shape of the curve which defines thetransmissivity of the device versus intensity characteristic of theelectromagnetic radiation). Also, this invention can function as asaturable absorber, as a saturable transmitter, or as a device havingcharacteristics between both of these. Another advantage of the presentinvention is its applicability to incident radiation having a wide rangeof wavelengths; for example, the present invention is usable with anylaser having a wavelength for which the components of the device aretransparent. This is not to imply that the The foregoing and otherobjects, features and ad- I vantages of the present invention willbecome more apparent in the light of the following detailed descriptionof preferred embodiments thereof as illustrated in the accompanyingdrawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 .is a simplified diagram of anonlinear optical transmission device in accordance with the presentinvention;

FIG. 2 is a simplified schematic diagram of a nonlinear opticalreflection device in accordance with the present invention; and

FIG. 3 is a plot for both a nonlinear saturable absorber andtransmitter, of the transmissivity as a function of intensity of anincident beam of electromagnetic radiation in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Various nonlinear opticaleffects including the saturation of an absorbing transition and theproduction of an induced absorption can provide intensity-dependenttransmission properties in selected materials. One such nonlinear effectis exploited in this invention and it relies on the intensity-dependentrotation experienced by the polarization ellipse of an ellipticallypolarized electromagnetic wave being propagated through certaindielectric materials. 7

The operation of this invention in all its embodiments relies on thisintensity-dependent rotation of an elliptically polarized wave. Theliquid materials of most interest include such materials as carbondisulfide, nitrobenzene and chlorobenzene and can be characterized asmaterials comprised of asymmetric or cigarshaped molecules which respondto a polarized wave passed therethrough by orienting their asymmetryabout a common axis. Alignment of the molecules changes the index ofrefraction of such dielectrics and the degree of alignment (andtherefore the degree of change in index of refraction) depends on theintensity of the wave. Thus, in an appropriate dielectric, the overalleffect is a material which not only exhibits an anisotropic index ofrefraction but also an index of refraction which changes with a changein the intensity of an electromagnetic wave transmitted therethrough.

In a typical cell containing a dielectric liquid, the index ofrefraction is not the same for right-hand and left-hand circularlypolarized light and the transmission of elliptically polarized radiationthrough such a cell causes the major axis of the ellipse to rotate. Therotation is proportional to the product of the electric fields along theprincipal axis of the polarization ellipse and the length of the liquidcell, and the direction in which this ellipse is rotated is dependentupon the sense of rotation of the ellipse.

The phenomenon of intensity-dependent rotation of a polarization ellipsehas been analyzed theoretically by Maker, PD. and Terhune, R.W., inStudy of Optical Effects Due to an Induced Polarization Third Order inthe Electric Field Strength, Physical Review I37, A801, (I965), anddemonstrated experimentally by Maker, P.D., Terhune R.W., and Savage,C.M., as described in Intensity-Dependent Changes in the RefractiveIndex of Liquids, Physical Review Letters 12,507 (1964). Under theconditions described in these materials, the angle of rotation of thepolarization ellipse, a, is given by the relationship a=2(rrm)/(cn)BE,,L (l) where (0 equals the frequency of the input optical radiation,

c is the velocity of light, it is the index of refraction of thedielectric,

B is a constant characteristic of the non-linearity of the index ofrefraction of the dielectric,

E is the component of electric field along one of the principal axes ofthe vibrational ellipse,

E, is the component of the electric field along the other of theprincipal axes of the vibrational ellipse, and

L is the length of the dielectric A nonlinear optical transmissiondevice is shown in FIG. 1 wherein a linearly polarized source or inputbeam 10 passes through a wave plate 12 which converts the linearlypolarized input into an elliptically polarized beam 14. The beam 14continues through a liquid cell 16 and interacts with the liquidcontained therein, causing the polarization axis of the beam 14 torotate and egresses the liquid cell as an elliptically polarized beam18, which has a different direction of orientation but the same degreeof ellipticity as the beam 14. The beam 18 is transmitted through asecond wave plate 20 and emerges as a beam 22 which may be elliptically,circularly or even linearly polarized depending upon the characteristicsof both the beam 18 and the wave plate 20. The beam passes through alinear polarizer 24, and emerges as an output beam 26 which is linearlypolarized along the allowed axis of polarization of the polarizer 24. Atransmission device of the type described is generally used outside of alaser optical cavity.

If it is assumed that in FIG. 1 the wave plates 12 and 20 are quarterwave plates and that the wave plate 12 has its fast axis oriented at anangle 0, to the direction of polarization of the input beam 10, that thewave plate 20 has its fast axis oriented at an angle 0, to the directionof polarization of the input beam 10, and that the polarizer 24 isoriented with its allowed axis at an angle 0, to the direction ofpolarization of the beam 10, the expression for the input beamtransmissivity (T) through such a device is given by plates 12 and willaccordingly change the transmissivity of the device in an amount whichis predetermined by the extent of the reorientation.

Referring: to FIG. 2 wherein a nonlinear optical reflection device isshown in schematic form, an input beam 30 passes through a linearpolarizer 32 emerging as a beam 34 which is linearly polarized along theallowed axis of the polarizer 32. The beam 34 passes through a waveplate 36 emerging as an elliptically polarized beam 38 which then passesthrough a liquid cell 40 emerging as a beam 42 which has the same degreeof ellipticity as the beam 38 although the direction of the major axisis different. The beam 42 passes through a second wave plate 44 strikinga mirror assembly 46 as a beam 48 and is reflected as a beam 50; themirror assembly may in fact be one mirror. The beam 50 passes throughthe wave plate 44 emerging as a beam 52 which may be circularly,elliptically or linearly polarized. The beam 52 passes through theliquid cell 40 emerging as a beam 54 which will be ellipticallypolarized if the input beam 52 had been elliptically polarized;alternatively if the input beam 52 was circularly or linearly polarizedthe output beam 54 would be correspondingly circularly or linearlypolarized. The beam 54 passes through the wave plate 36 emerging as abeam 56 whose polarization properties depend on the precise propertiesof the input beam 54 and the wave plate 36. The beam 56 passes throughthe polarizer 32 emerging as an output beam 58 which is linearlypolarized along the allowed axis of the polarizer 32. Although the beampath from left to right is shown as physically separated from the beampath going from right to left in FIG. 2, these beam paths can and dooverlap. A reflection device of the type just described is typicallyused inside a laser optical cavity with the mirror 46 functioning as oneof the cavity mirrors.

Assuming that in the reflection device shown in FIG. 2 the wave plates36 and 44 are quarter wave plates, that the input beam 30 is linearlypolarized along the allowed axis of the polarizer 32, that the fast axisof the wave plate 36 is at an angle 0, to the direction of polarizationof the input beam 30, and that the fast axis of the wave plate 44 is atan angle 0, to the direction of polarization of the input beam 30, thenthe reflectivity (R) is given by the equation where"=((21r))/().)LBE,sin20 By analogy to the example given for thetransmission device described by Equation (2), it should be apparentthat reorientation of the fast axis of either of the wave plates 36 or44 results in a correspondingly predeterminable change in reflectivity.

The transmissivity of nonlinear optical devices in accordance with thepresent invention is a function of the intensity of the input beam; thisintensity dependency is reflected in Equations (2) and (4), both ofwhich incorporate a which is also intensity-dependent as is indicated inEquations (3) and (5), respectively. The transmissivity of these devicesis further dependent upon the orientation of the wave plates and thepolarizers as is reflected in Equations (2) and (4). In any givensystem, a wide range of transmissivity versus intensity characteristicsis available by appropriate I s 1+ (6) the transmissivity (T) is givenby the equation T l-cos20,sin'(0,-[a-l-0 (7) The nonlinear variation intransmissivity of devices assembled according to the present inventionis a periodic function (as shown in FIG. 3), although only the firstcycle would generally be utilized.

A transmission device having saturable absorber type characteristics, asillustrated in FIG. 3 by Curve A, can be realized if the orientationsare such that 0,-0, n/(2a)- Similarly, a transmission device havingsaturable transmitter characteristics, as illustrated in FIG. 3 by CurveB, can be realized if the orientations are such that 0,-0, 0.Characteristic forms intermediate the saturable transmitter and thesaturable absorber can be realized by variations in the orientations 0;and 0,.

It should be evident that a wide range of transmissivity characteristicscan be obtained for both saturable absorber and saturable transmitterdevices by alternate choices of the orientation of the optical elementsand the length of the liquid cell. The maximum and minimumtransmissivity, the periodicity, and the location of the maximum andminimum with respect to the zero of intensity can be adjusted asdesired.

There is a corresponding degree of freedom with respect to reflectiondevices as has been illustrated above with respect to transmissiondevices.

As an example of the magnitude of the electric fields typically requiredto produce a transmissivity change, the quantity or as given in Equation(1) is calculated as follows. From data published by PD. McWane and D.A.Sealer, New Measurements of the Intensity Dependent Changes in theRefraction Index of Liquids, Applied Physics Letter 8, p.278-279, l June1966,

for carbon disulfide at a wavelength of 6943 A. Assuming a cell lengthof one centimeter, then from Equation (1 E sin 28;

Since the transmissivity of the transmission device is periodic in awith a period of 1r, the field required to cover one period of thecharacteristic is n=( X 10' and E, 0.72 X 10 stat volts/cm.

which corresponds to a power level of approximately 60 Megawatts/cm,typical of the saturation intensity of conventional saturable absorbers.

Although the invention has been described with respect to preferredembodiments thereof, it should be understood to those skilled in the artthat the foregoing and other changes in the form and detail thereof canbe made therein without departing from the spirit and the scope of theinvention.

Having thus described typical embodiments of my invention, that which iclaim as new and desire to secure by Letters Patent of the United Statesis:

1. An optical device which transmits an input beam of linearly polarizedelectromagnetic radiation and has a transmissivity characteristic whichis a function of the intensity of the input beam comprising:

a first quarter wave plate with its fast axis oriented at an angle tothe direction of polarization of the input beam for producing from theinput beam an elliptically polarized beam having a particularorientation;

a liquid dielectric material which rotates the orientation of saidelliptically polarized beam by an angle a producing a rotatedelliptically polarized beam;

a second quarter wave plate with its fast axis oriented at an angle 0,to the direction of polarization of the input beam for producing asecond linearly polarized beam from said rotated elliptically polarizedbeam; and

a linear polarizer oriented with its allowed axis at an angle 0 to thedirection of polarization of the input beam which passes only thatcomponent of the second linearly polarized beam which lies along theallowed axis so that said optical device has an overall transmissivity(T) described by 2. An optical device which reflects a linearlypolarized input beam of electromagnetic radiation and has reflectivitycharacteristics which depend upon the intensity of the input beamcomprising:

a linear polarizer for transmitting that component of the input beamwhich is polarized along a preselected axis with its allowed axis at thesame orientation as the direction of the polarization of the input beam;

a first quarter waveplate with its fast axis oriented at an angle 0 tothe direction of polarization -of the input beam for ellipticallypolarizing that component of the input beam which is passed by thelinear polarizer to produce an elliptically polarized beam having amajor and a minor axis of polarization;

a cell containing liquid dielectric material which rotates theorientation of the axes of the polarization ellipse by an angle a" whichis dependent upon the intensity of the input beam to produce a V thesecond waveplate, the liquid dielectric, the first waveplate and thelinear polarizer so that the optical device has an overall reflectivity(R) described by:

3. A method of producing an output beam of linearly polarized radiationincluding the steps of:

providing an input beam of electromagnetic radiation;

converting the input beam to an elliptically polarized beam;

rotating the orientation of the polarized beam in an amount whichdepends upon the intensity of the polarized beam to produce a rotatedelliptically polarized beam;

altering the polarization of the rotated beam; and passing only thatcomponent of the altered polarization beam which lies along apreselected axis of polarization whereby the output beam is a polarizedbeam of radiation whose intensity is a nonlinear function of theintensity of the input beam.

4. A method of providing an output beam of polarized radiation includingthe steps of:

providing an input beam of electromagnetic radiation;

converting the input beam to an elliptically polarized beam;

rotating the orientation of the polarized beam in an amount whichdepends upon the intensity of the polarized beam to produce a rotatedelliptically polarized beam;

altering the polarization of the rotated beam and reversing thedirection of travel;

further rotating the orientation of the polarized beam in an amountwhich depends upon the intensity of the polarized beam to produce afurther rotated elliptically polarized beam;

further altering the polarization of the beam; and

selecting a component of linear polarization from the beam whereby theoutput beam is a polarized beam of radiation whose intensity is anonlinear function of the intensity of the input beam.

i i i t g UIHTED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,700,307 0' Dated October 24; 1972 Inven r( William H. Glenn It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby cotprected as shown below:

Column 4, Line 10, change [=2()Tw /(cn) BE L' to d. =27?! BE E L/(cn) 2y Line 57, change "sin( 95-92%)" to sin (9 9 +9 Line 58, change )sin(9TfL -[d 9 to 5511 092 a 0 q. I

1 Line 60, change d ((2 )QLBE inZQ to a '=(27T) LBE sin29 V Column 5,Line 51, change "d"=( (2 (A )LBEO SiHZQl" to a= 27t LBE- 251E129 eColumn 6, Line 17 andll8 change "9 -9 1=(3T)/(2a)' to Line 46,change"6943 'A" to 6943 2 Line 56, change "BE 0' to E0 Column 7, Line 21,change Q: to at Line 34, change "sin( 93-9 +9" to t 2 r 2 sin 9 9 +9Line 35, change )sin (9 to sin (9 Signed and sealed this 3rd day ofApril 1973..

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

1. An optical device which transmits an input beam of linearly polarizedelectromagnetic radiation and has a transmissivity characteristic whichis a function of the intensity of the input beam comprising: a firstquarter wave plate with its fast axis oriented at an angle theta 1 tothe direction of polarization of the input beam for producing from theinput beam an elliptically polarized beam having a particularorientation; a liquid dielectric material which rotates the orientationof said elliptically polarized beam by an angle Alpha producing arotated elliptically polarized beam; a second quarter wave plate withits fast axis oriented at an angle theta 2 to the direction ofpolarization of the input beam for producing a second linearly polarizedbeam from said rotated elliptically polarized beam; and a linearpolarizer oriented with its allowed axis at an angle theta 3 to thedirection of polarization of the input beam which passes only thatcomponent of the second linearly polarized beam which lies along theallowed axis so that said optical device has an overall transmissivity(T) described by T cos2( theta 3- theta 2-1)cos2( theta 2-( Alpha ''+theta 1))+sin2( theta 3- theta 2+ theta 1)sin2( theta 2-( Alpha ''+theta 1)).
 2. An optical device which reflects a linearly polarizedinput beam of electromagnetic radiation and has reflectivitycharacteristics which depend upon the intensity of the input beamcomprising: a linear polarizer for transmitting that component of theinput beam which is polarized along a preselected axis with its allowedaxis at the same orientation as the direction of the polarization of theinput beam; a first quarter waveplate with its fast axis oriented at anangle theta to the direction of polarization of the input beam forelliptically polarizing that component of the input beam which is passedby the linear polarizer to produce an elliptically polarized beaM havinga major and a minor axis of polarization; a cell containing liquiddielectric material which rotates the orientation of the axes of thepolarization ellipse by an angle which is dependent upon the intensityof the input beam to produce a rotated elliptically polarized beam; asecond quarter waveplate with its fast axis oriented at an angle theta 2to the direction of polarization of the input beam for further alteringthe polarization state of the rotated elliptically polarized beam whichemits an altered polarization beam; and a fully reflecting mirror forreflecting the altered polarization beam emitted by the second waveplateso that the beam passes in order through the second waveplate, theliquid dielectric, the first waveplate and the linear polarizer so thatthe optical device has an overall reflectivity (R) described by: R1-cos22 theta 1sin22( theta 2- Alpha '''' theta 1).
 3. A method ofproducing an output beam of linearly polarized radiation including thesteps of: providing an input beam of electromagnetic radiation;converting the input beam to an elliptically polarized beam; rotatingthe orientation of the polarized beam in an amount which depends uponthe intensity of the polarized beam to produce a rotated ellipticallypolarized beam; altering the polarization of the rotated beam; andpassing only that component of the altered polarization beam which liesalong a preselected axis of polarization whereby the output beam is apolarized beam of radiation whose intensity is a nonlinear function ofthe intensity of the input beam.
 4. A method of providing an output beamof polarized radiation including the steps of: providing an input beamof electromagnetic radiation; converting the input beam to anelliptically polarized beam; rotating the orientation of the polarizedbeam in an amount which depends upon the intensity of the polarized beamto produce a rotated elliptically polarized beam; altering thepolarization of the rotated beam and reversing the direction of travel;further rotating the orientation of the polarized beam in an amountwhich depends upon the intensity of the polarized beam to produce afurther rotated elliptically polarized beam; further altering thepolarization of the beam; and selecting a component of linearpolarization from the beam whereby the output beam is a polarized beamof radiation whose intensity is a nonlinear function of the intensity ofthe input beam.